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.TH FENV.H 3HEAD "September 22, 2020"
.SH NAME
fenv.h, fenv \- floating-point environment
.SH SYNOPSIS
.nf
#include <\fBfenv.h\fR>
.fi

.SH DESCRIPTION
The <\fBfenv.h\fR> header defines the following data types through
\fBtypedef\fR:
.sp
.ne 2
.na
\fB\fBfenv_t\fR\fR
.ad
.RS 13n
Represents the entire floating-point environment. The floating-point
environment refers collectively to any floating-point status flags and control
modes supported by the implementation.
.RE

.sp
.ne 2
.na
\fB\fBfexcept_t\fR\fR
.ad
.RS 13n
Represents the floating-point status flags collectively, including any status
the implementation associates with the flags. A floating-point status flag is a
system variable whose value is set (but never cleared) when a floating-point
exception is raised, which occurs as a side effect of exceptional
floating-point arithmetic to provide auxiliary information. A floating-point
control mode is a system variable whose value can be set by the user to affect
the subsequent behavior of floating-point arithmetic.
.RE

.sp
.LP
The <\fBfenv.h\fR> header defines the following constants if and only if the
implementation supports the floating-point exception by means of the
floating-point functions \fBfeclearexcept()\fR, \fBfegetexceptflag()\fR,
\fBferaiseexcept()\fR, \fBfesetexceptflag()\fR,  and \fBfetestexcept()\fR. Each
expands to an integer constant expression with values such that
bitwise-inclusive ORs of all combinations of the constants result in distinct
values.
.sp
.in +2
.nf
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
.fi
.in -2

.sp
.LP
The <\fBfenv.h\fR> header defines the following constant, which is simply the
bitwise-inclusive OR of all floating-point exception constants defined above:
.sp
.in +2
.nf
FE_ALL_EXCEPT
.fi
.in -2

.sp
.LP
The <\fBfenv.h\fR> header defines the following constants. Each expands to an
integer constant expression whose values are distinct non-negative values.
.sp
.in +2
.nf
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
.fi
.in -2

.sp
.LP
The <\fBfenv.h\fR> header defines the following constant, which represents the
default floating-point environment (that is, the one installed at program
startup) and has type pointer to const-qualified \fBfenv_t\fR. It can be used
as an argument to the functions within the <\fBfenv.h\fR> header that manage
the floating-point environment.
.sp
.in +2
.nf
FE_DFL_ENV
.fi
.in -2

.sp
.LP
The \fBFENV_ACCESS\fR pragma provides a means to inform the implementation when
an application might access the floating-point environment to test
floating-point status flags or run under non-default floating-point control
modes. The pragma occurs either outside external declarations or preceding all
explicit declarations and statements inside a compound statement. When outside
external declarations, the pragma takes effect from its occurrence until
another \fBFENV_ACCESS\fR pragma is encountered, or until the end of the
translation unit. When inside a compound statement, the pragma takes effect
from its occurrence until another \fBFENV_ACCESS\fR pragma is encountered
(including within a nested compound statement), or until the end of the
compound statement; at the end of a compound statement the state for the pragma
is restored to its condition just before the compound statement. If this pragma
is used in any other context, the behavior is undefined.
.sp
.LP
If part of an application tests floating-point status flags, sets
floating-point control modes, or runs under non-default mode settings, but was
translated with the state for the \fBFENV_ACCESS\fR pragma off, the behavior
is undefined. The default state (on or off) for the pragma is
implementation-defined. (When execution passes from a part of the application
translated with \fBFENV_ACCESS\fR off to a part translated with
\fBFENV_ACCESS\fR on, the state of the floating-point status flags is
unspecified and the floating-point control modes have their default settings.)
.SH USAGE
This header is designed to support the floating-point exception status flags
and directed-rounding control modes required by the IEC 60559: 1989 standard,
and other similar floating-point state information. Also, it is designed to
facilitate code portability among all systems.  Certain application programming
conventions support the intended model of use for the floating-point
environment:
.RS +4
.TP
.ie t \(bu
.el o
A function call does not alter its caller's floating-point control modes, clear
its caller's floating-point status flags, or depend on the state of its
caller's floating-point status flags unless the function is so documented.
.RE
.RS +4
.TP
.ie t \(bu
.el o
A function call is assumed to require default floating-point control modes,
unless its documentation promises otherwise.
.RE
.RS +4
.TP
.ie t \(bu
.el o
A function call is assumed to have the potential for raising floating-point
exceptions, unless its documentation promises otherwise.
.RE
.sp
.LP
With these conventions, an application can safely assume default floating-point
control modes (or be unaware of them). The responsibilities associated with
accessing the floating-point environment fall on the application that does so
explicitly.
.sp
.LP
Even though the rounding direction macros might expand to constants
corresponding to the values of \fBFLT_ROUNDS\fR, they are not required to do
so. For example:
.sp
.in +2
.nf
#include <fenv.h>
void f(double x)
{
    #pragma STDC FENV_ACCESS ON
    void g(double);
    void h(double);
    /* ... */
    g(x + 1);
    h(x + 1);
    /* ... */
}
.fi
.in -2

.sp
.LP
If the function \fIg\fR() might depend on status flags set as a side effect of
the first \fIx\fR+1, or if the second \fIx\fR+1 might depend on control modes
set as a side effect of the call to function \fIg\fR(), then the application
must contain an appropriately placed invocation as follows:
.sp
.in +2
.nf
#pragma STDC FENV_ACCESS ON
.fi
.in -2

.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
.TE

.SH SEE ALSO
.BR feclearexcept (3M),
.BR fegetenv (3M),
.BR fegetexceptflag (3M),
.BR fegetround (3M),
.BR feholdexcept (3M),
.BR feraiseexcept (3M),
.BR fesetenv (3M),
.BR fesetexceptflag (3M),
.BR fesetround (3M),
.BR fetestexcept (3M),
.BR feupdateenv (3M),
.BR attributes (7),
.BR standards (7)
